UFC Undisputed 3 is coming out too and that looks amazing
Anderson Silva is an asshole, but god dammit he's an amazing amazing fighter. Unfortunately I don't think there's anyone out there who can give him a legitimate challenge, unless GSP went up a class to fight him (which would be the greatest fight in the *short* history of Mixed Martial Arts).
GSP is ridiculous. He is not only an amazing fighter, but keeps himself in peak physical condition all the time. I have never seen him look anything less than 100% prepared for a fight.
This. GSP is without doubt the fighter who's constantly the most prepared. Unbelievable fitness, and great technique. He's just... a little boring to watch. He never really goes all out; fetching points rather than entertainment - which is fair enough, it just leads me to usually enjoying other fights in the night more.
There are so many great fighters at all the different weight classes (so glad they brought in bantam and feather-weight), that it's hard to choose specifically. I was a big fan of Diego Sanchez since his run on TUF. Impeccable determination, and he's just a great, great fighter to watch. That fight with Clay Guida a couple of years back was amazing. He hasn't been on such top form for a while, though, and sat on his laurels a little. Showed him up against his fight with the young brit last year (I forget his name)...
Favorite of all time?
Probably Sakuraba. The dude was a super nerd. It's also hilarious how he became the guy known for beating the Gracie family. He's been semi-retired for a bit, so I'm going to view him as not current.
Currently fighting, I really don't have a 'favorite'. Usually I'll find some sort of superficial reason to root for one fighter over another. Donald Cerrone is always entertaining.